Fix #84 - slice outgoing messages according to frameMax

Content frames are ignoring the frame max setting, and just send the entire body as one frame.  (see #84)

This commit does three things:
 1) amends `maxFrameBuffer` to the value advertised by the server
 2) extracts main logic from `_sendBody` function to a  `_bodyToBuffer`
 3) `_sendBody` is rewritten in a simpler fashion, assuming it only deals with buffers (non-buffers are being converted to a buffer in `_bodyToBuffer`) 

I don't know how to run the tests from `test` directory, I verified this code manually.